Abstract: The unequal distribution of longitudinal magnetic field in plasma and its influence on the arc shape of TIG are discussed in detail based on single-particle theory.The arc force with the longitudinal magnetic field is analyzed,and the motion equation of the charged particle in plasma with the unequal distribution of the longitudinal magnetic field is established.The path of the particle in the magnetic field is a convergence helix.The circular motion speed of particles in the external of plasma arc is much more than that in the column central of the arc.The experiments show that TIG arc is compressed obviously under magnetic field,and when exciting current is constant,the higher the magnetic frequency is,the more severely the arc is condensed.